This invention relates to a polymer composition having reduced fluorescence, and particularly to a polymer blend composition containing naphthalenedicarboxylic acid residues and having incorporated therein benzylidene compounds. The invention further relates to a method for preparing the blend.
It is well known that dicarboxylic acids and dicarboxylic esters can be reacted with glycols to prepare polyesters. Polyesters are used in the fabrication of various articles for household, commercial or industrial use. Such uses included appliance parts, containers, and auto parts. A major drawback of many polyesters is their inherent bluish fluorescence. For example, articles prepared with the polyester poly(ethylene-2,6-naphthalene dicarboxylate), also known in the art as "PEN", have a hazy and bluish appearance. This phenomenon is of concern especially in packaging of products when their appearance is to be as close to their natural state as desired. For example, in the packaging of foods and beverages, if the food or beverages were inside a PEN container they may appear unnatural.
Typically, poly(ethylene 2,6-naphthalene dicarboxylate) contains repeat units from a dicarboxylic acid and a diol. The poly(ethylene 2,6-naphthalene dicarboxylate) can be prepared by conventional polycondensation procedures well-known in the art. Such procedures can include a combination of melt phase and solid state polymerization. Melt phase describes the molten state of PEN during the initial polymerization process. The initial polymerization process includes direct condensation of the naphthalene-2,6-dicarboxylic acid with the diol(s) or by ester interchange using naphthalene-2,6-dicarboxylic ester. The melt phase is concluded by extruding the PEN polymer into strands and pelletizing. Solid state polymerization involves heating PEN pellets to a temperature in excess of 200.degree. C., but well below the crystalline melt point, either in the presence of an inert gas stream or in a vacuum to remove a diol.
Fluorescence is a type of luminescence in which an atom or molecule emits radiation, i.e., a photon, in passing from a higher to a lower electron state. The term is restricted to phenomena in which the time interval between absorption and emission energy is extremely short (10.sup.-10 to 10.sup.-6 second). Quenching fluorescence eliminates or reduces the ability for photon emission by providing an alternative pathway for the excited state energy such as vibronic or heat loss, or intersystem crossing to the excited triplet state. Methods to quench fluorescence in PEN have been disclosed by Chen Shangxian, et al. in an article entitled, "Fluorescence Spectra of Poly(Ethylene 2,6-Naphthalene Dicarboxylate)" which appeared in SCIENTIA SINICA, Vol. XXIV, No. 5, May 1981, and by CAO Ti, et al. in an article entitled, "Intermolecular Excimer Interaction in Poly(Polytetramethylene Ether Glycol Aryl Dicarboxylate)" which appeared in ACTA CHIMICA SINICA, Vol. 42, No. 1, 1984. Both of the references disclose the use of o-chlorophenol to quench PEN fluorescence in a chloroform solution. Dissolving PEN in a chloroform solution to disperse the fluorescence quencher therein, however, is not practical since the PEN must have a low molecular weight to dissolve in the chloroform solution and only very dilute PEN solutions can be prepared.
Japanese Patent 08225672 discloses the use of coatings containing UV absorbers benzotriazoles, cyanoacrylates, benzophenones, and benzoxazinones to suppress the fluorescence of a molded product consisting mainly of polyalkylenenaphthalene-2,6-dicarboxylate. A disadvantage of using a coating to reduce fluorescence of PEN is the additional processing steps required to apply the coating to the molded article. The cost of coating adds significantly to the cost of the article.
European Patent 0711803 discloses mixing cyclic imino esters or quinoxalines with PEN as a method of reducing fluorescence in the polyalkylenenaphthalene-2,6-dicarboxylate. Blending the cyclic imino esters and quinoxalines with the polyalkylenenaphthalene-2,6-dicarboxylates resulted in moderate changes in the relative fluorescence intensity (37% to 58%) which is not enough to significantly improve the appearance of containers made from polymer compositions containing naphthalenedicarboxylic acid residues. European Patent 0711803 further discloses using UV absorbing coatings in combination with the above blends to reduce the relative fluorescence intensity to an adequate level.
U.S. Pat. Nos. 4,617,374 and 4,707,537 disclose the use of various benzylidene compounds as UV screens for polyesters. These patents teach that incorporating the benzylidene compounds in various polyesters at a concentration of 1 to 5000 ppm with a maximum absorbance of 320 nm to about 380 nm is effective in screening UV light. The patents do not disclose the use of the benzylidene compounds in polymer compositions containing naphthalenedicarboxylic acid residues. Polymer compositions containing naphthalenedicarboxylic acid residues such as poly(ethylene 2,6-naphthalenedicarboxylate) can absorb UV light up to 390 nm, thus, making UV absorbers ineffective as UV screens. U.S. Pat. Nos. 4,617,374 and 4,707,537 are unclear as to the effect the benzylidene compounds would have on the fluorescence intensity of polymer compositions containing naphthalenedicarboxylic acid residues.
U.S. Pat. No. 5,310,857 discloses reducing the fluorescence of a poly(ethylene 2,6-naphthalenedicarboxylate) polymer using from 0.1 to 5 mole percent of a copolymerizable aromatic ketone. The aromatic ketone has at least one acyl group directly attached to the aromatic ring.
U.S. Pat. No. 5,352,761 discloses certain naphthalene derivatives having substituted aroyl groups which are useful in providing reduced fluorescence in polyesters such as PEN and are copolymerizable with other polyesters.
U.S. Pat. No. 5,391,701 discloses using difunctional halo-aromatic compounds copolymerized with PEN to reduce the polymer's fluorescence.
U.S. Pat. Nos. 5,391,702 and 5,391,330 disclose a process for preparing PEN or dimethyl 2,6-naphthalene dicarboxylate containing copolyesters/fluorescence quencher blends via melt blending.
U.S. Pat. No. 5,393,862 discloses using nonhalogenated copolymerizable aromatic compounds such as benzene, naphthalene and biphenyl having at least one acyl group directly attached to the aromatic ring to reduced the fluorescence in polymers of dimethyl 2,6-naphthalene dicarboxylate.
U.S. Pat. No. 5,418,318 discloses the use of dimethyl 2,6-naphthalene dicarboxylate copolyesters or copolyamides having a copolymerized halo-aromatic compound and having reduced fluorescence.
Accordingly, there is a need for poly(ethylene 2,6-naphthalene dicarboxylate) having a reduced fluorescence without deleteriously affecting the physical properties of the polymer.